Parasites in the Bloodstream
A variety of parasite species, including protozoans and
helminthes are found in human blood at some stage in their life cycle. These
include species of malaria, trypanosomes, babesias and microfilariae and several
species of filarial nematodes. The characteristic stage of each found is either
moving freely among the blood cells or inhabiting the red blood cells as
intracellular parasites. When present in sufficient numbers, organisms may be
found in a single drop of blood. Often, however, their numbers are so small that
they will be found only in large or concentrated blood samples.
- It is recommended to collect fingerstick blood droplets on to
a microscope slide at the bedside.
- For confirmation and thick smear examination, lavender (EDTA)
topped tubes are collected via venipuncture.
- Skin Puncture
- Site selection: Finger puncture using the palmar surface of the tip of the fingers are preferred
for blood films for parasites. The middle or "ring" fingers are generally used.
- Warming the site: The skin area to be punctured should be warmed to increase the blood
flow. Depending on the physical setting and the patient's condition, warming the
hands can be done by immersing in hot water, by briskly rubbing the area or by
covering with a hot moist towel
- Cleaning the site: The puncture site should be cleaned and disinfected
with gauze squares or commercial non-cotton alcohol preparations soaked
in 70% alcohol, then wiped dry with sterile gauze or allowed to air-dry prior to
puncture. Cotton should not be used to clean the finger prior to skin puncture
because loose fibers may lodge in the blood, resulting in confusing artifacts.
- The fingers must be dry before sticking so that the drop of blood will
round up on the finger and not run down the finger or hand. Also, any mixture of
remaining alcohol with the blood will "fix" the red cells, thus making the thick
films unsuitable for staining.
- Technique: The technique for puncturing the finger is to stick the finger
with a sterile, non-reusable lancet deeply enough to allow collection of a
sufficient amount of free-flowing blood to prepare thick and thin films. After
collection of the specimen, pressure should be applied to the puncture site and
sterile cotton or sterile gauze until bleeding stops. Then a bandage should be
applied to the puncture site.
- Recommendation for use: Unless it is certain that well prepared blood
smears will not be received, venipuncture is usually not recommended for
obtaining blood for films for parasite examinations (malaria, Babesia
or hemoflagellates). Since Plasmodium falciparum shizonts concentrate in
the microvasculature, skin puncture thick and thin films have a higher
sensitivity than venipuncture.
- Use of Anticoagulants: Sometimes the anticoagulants may interfere with
adhesion of the blood to the slide during smear preparation and with proper
staining of the parasites. This is particularly true with the ratio of blood to
anticoagulant is incorrect in the collection tube (too small amount of blood per
amount of anticoagulant).
Timing of blood collection
- BLOOD SHOULD BE COLLECTED IMMEDIATELY UPON SUSPICION OF MALARIA,
although the optimum time is about midway between chills to ensure obtaining
stages on which species identifications can be made.
- Since single films may not reveal organisms, successive films at 6, 12 or
24 hours are sometimes necessary.
- Blood samples must be taken before any anti – malarial drugs are used to
ensure demonstration of organisms if the patient does have malaria.
- AT THE TIME OF ADMISSION OF THE PATIENT, four blood films, 2 thin
and 2 thick (one slightly thinner than usual, one regular) are prepared.
Other blood parasites:
- Blood samples should be collected in the early phases of the
disease (within one month) for optimal recovery.
- Contact the Microbiology laboratory for more information on
optimal time to collect specific parasites.
Preparing blood smears/films:
- Thin films – A well-prepared film is thick on one end and thin at the other (one
layer of evenly distributed RBC's with no cell overlap). The thin, feathered end
should be at least 2cm long, and the film should occupy the central area of the
slide, with free margins on each side.
- Place one drop (approximately 0.05 ml) of blood near one end of a glass microscope slide.
- Hold a second, narrower spreader slide with polished edges at a 45º angle and
immediately draw into the drop of blood. Allow the blood to spread almost to the
width of the slide.
- Rapidly and smoothly push the spreader slide to the opposite end of the slide, pulling
the blood behind it.
- Thick films – Thick films should be at least the size of a
dime or nickel (1.5 to 2.0 cm diameter) and thick enough to just read newspaper
print through it. Films may be prepared by either "contact" method or "puddle
- Touch the slide to the drop of blood (which should be rounded up on the finger).
- Rotate the slide to form a circular film of the appropriate
size and density. If the drop is not large enough or the blood begins to clot,
this method will not work well.